In the prior art, sulphuric acid is usually prepared on an industrial scale by the double contact process in a reactor having one or more catalyst-containing contact stages (as described, for example, in Ullmanns Encyclopedia of Industrial Chemistry). The oxidation SO2 +½O2−>SO3 usually occurs catalytically, e.g. over a vanadium pentoxide catalyst with or without cesium in a temperature window of T=380° C.-650° C. Furthermore, it is known that at T<340° C.-380° C. the catalyst does not reach the required ignition temperature and that it is irreversibly damaged at T>630° C.-650° C. It is also known that at an SO2 content of above 13.5% by volume the reaction in a classical contact stage is so exothermic that the catalyst forms hot spots and is irreversibly damaged. To meet the requirements of the catalyst, plants corresponding to the prior art are therefore operated at T<640° C. and SO2 contents of <13.5% by volume in the starting gas to avoid irreversible damage to the catalyst. In addition, to ensure that the catalyst starts working in a process carried out adiabatically according to the prior art, the starting gas inlet temperature has to be T>380° C. and the SO2 contents have to be > about 5% by volume.
For the catalytic oxidation of starting gases having an SO2 content of >13.5% by volume, it has been proposed, for example, that the starting gas firstly be diluted with air before introduction to the catalyst and that correspondingly greater gas volumes be passed through the plant. Particularly when using pyrometallurgical offgases as sulphur dioxide-rich starting gases having SO2 contents of 20-66% by volume, a large dilution factor would be necessary, resulting in disproportionately high costs.
DE-A 2026818 (corresponding to U.S. Pat. No. 3,755,549) describes a process in which the starting gases are diluted with SO3 driven off from oleum and additional dilution air to an SO2 concentration of 10-20% by weight before entering the first contact stage. Disadvantages are the high costs and the low specific SO2 conversion in the first contact stage.
DE-10249782 (corresponding to US 2006/245,997) describes a process for preparing sulphuric acid on the basis of concentrated starting gases having an SO2 concentration of >13.5% by volume in a reactor having a plurality of contact stages, which can be operated using relatively small amounts of dilution air. The dilution in the first contact stage is achieved by recirculation of a partially oxidised gas stream. A substream of the SO2/SO3-containing gas is taken off from a contact stage preceding the last main contact stage. This substream is mixed with the starting gas (SO2 content of more than 13% by volume) to form a contact gas which has an SO2 content of <13% by volume to reliably avoid overheating of the catalyst and can be fed to the first contact stage. Here, the thermodynamic equilibrium of the reaction SO2+½O2<=>SO3 is shifted in the direction of the starting materials, thus avoiding overheating of the catalyst. A disadvantage of this process is the high outlay in terms of apparatus.
Particularly in metallurgical processes and the resulting offgasses but also for increasing the capacity of existing sulphuric acid plants using sulphur combustion, it is necessary to have economical processes for the treatment of starting gases having SO2 concentrations of >13.5% by volume while maintaining the volume flows and thus the hydraulics.
In many chemical processes in which sulphur compounds are used, an economical and ecological process for the treatment of starting gases having an SO2 content of <5% by volume even with strongly fluctuating concentrations is also required.
It is therefore an object of the invention firstly to make possible the inexpensive preparation of sulphuric acid for concentrated starting gases having sulphur dioxide contents of >13.5% by volume and secondly to provide an economically ecological process for sulphur dioxide-containing offgases (SO2<5% by volume) from various chemical processes.